Method and apparatus for forming longitudinal joints in concrete

ABSTRACT

A method and apparatus for forming a longitudinal joint in a concrete surface. A vertical knife is mounted on a concrete slip-forming machine and extends to a predetermined depth in the concrete surface. As the knife is pulled through the formed concrete by the machine, aggregate or reinforcing fibers are deflected to either side of the knife. An aggregate- or fiber-free strip of concrete paste fills in the gap behind the knife creating a weakened zone which acts to constrain the formation of cracks in the concrete surface to the formed joint. The longitudinal joint in the roadway is formed without sawing, cleaning, or sealing and does not require maintenance.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to a method and apparatus for forminglongitudinal joints in concrete as it is being slip-formed and, morespecifically, to a method for using a knife beneath a slip-formingmachine, such as for slip-forming a section of concrete roadway, thatforms a longitudinal joint in the roadway that does not require sawing,cleaning, sealing or maintenance.

2. Background of the Prior Art

It has become very common to use slip-forming techniques in the formingof concrete roadways. A machine, such as that described in U.S. Pat. No.4,073,592, is used to form one or more lanes of a concrete roadway byslip-forming. Wet or plastic concrete is added to the machine and it isformed into the desired roadway configuration by a plurality of formingsurfaces. The concrete is applied by the machine to a prepared road bed,typically including reinforcing steel to strengthen the formed roadway.In a multiple lane roadway, reinforcing steel is used to span the lanesin a transverse direction to tie the lanes together. The reinforcingsteel is effective at preventing lateral separation of the lanes butallows some degree of flexure along the joint between the lanes.

Cured concrete expands and contracts with changes in temperature. Theexpansion and contraction can lead to the formation of cracks whichweaken and lead to breaking of the concrete. The larger the unitarystructure of the concrete, the more likely it will crack due toexpansion and contraction. Accordingly, one method of preventing theformation of cracks, at least in areas where cracks are not desired, itto form the concrete into reduced size sections that will be moreresistant to cracking. In forming a roadway, if more than one lane isformed at a time, the width of the multiple lanes is typically too greatto be left as a unitary structure. The approach to preventing undesiredcracking has been to form a longitudinal joint down the formed roadwayto divide the roadway into multiple lanes. The joint provides a placefor concrete expansion, contraction or flexural movements to occur in astraight line and controlled manner.

The conventional method of constructing a longitudinal joint in aroadway paving project comprises four distinct operations. After theplastic concrete has been placed and molded, it is allowed to hardenjust sufficiently to carry the load of a concrete saw and associatedequipment. The first step is sawing a longitudinal joint into thehardened concrete. The joint is typically between about one-eighth andthree-eighths inch wide and to a depth of ⅓ of the thickness of thepavement being formed. Next, the joint is blown clean using a highcapacity air compressor. The third operation, usually done twenty-fourto forty-eight hours later, is installing a foam backer rode or ropeinto the joint down to about one inch below the pavement surface tocreate a base. The final step is to fill the joint reservoir above thebacker rod with a sealant such as a hot, liquid asphaltic rubbersealant. This process requires the use of a saw, blades that can cutconcrete and the aggregate used in the concrete, a lubricant, air orwater cleaning of the groove, a drying period, insertion of the backerrod, placement of the sealant, and periodic maintenance to repair thesealant. Accordingly, this practice is expensive, noisy, dirty, andtime-consuming.

A need exists for a method of forming longitudinal joints in a roadwaythat does not require post-forming processing, is inexpensive,noiseless, clean, fast, and without requiring periodic maintenance.

SUMMARY OF THE INVENTION

The invention consists of a method and apparatus for forminglongitudinal joints in concrete as it is being slip-formed. A knifemember having a depending knife is supported in a floating relationshipon the surface of the roadway being formed. The knife extends below thesurface of the concrete. As the slip-forming machine travels forwardly,the knife member travels longitudinally across the surface of theformed, uncured concrete. The knife forms a groove in the concrete.Preferably, the knife member also includes a horizontal float thatassists in smoothing the surface of the disturbed concrete. Alsopreferably, the knife member and associated knife are vibrated as theknife forms the groove to assist in the operation of the knife andsmoothing of the concrete surface by the horizontal float of the knifemember.

The knife member is positioned under the slip-forming machine andaligned to form the joint, most generally coinciding with the centerlineof the roadway being formed. The knife member can be mounted eitherforwardly or rearwardly on the machine, provided that the knife extendsinto the plastic concrete. The knife pushes aside each piece ofaggregate encountered in its path, creating a longitudinally extended,vertical strip of concrete paste. This strip is a plane of weakness anda straight line crack or joint later occurs in the hardened concretealong the weakened plane, thereby forming an induced crack or joint. Thejoint appears as a longitudinal hairline crack. The joint is heldtightly closed by deformed steel reinforcing tie bars positionedtransversely at mid depth in the concrete at periodic spacing along thejoint, as in the conventional roadway design. Sealing of this tightjoint is not usually needed.

An object of the invention is to provide a method for forming alongitudinal joint in concrete that is performed in a single pass as theconcrete is being formed.

Another object of the invention it to provide apparatus for forming alongitudinal joint in concrete that readily mounts on existing concreteforming machinery and is operable concurrently with such machinery.

A further object of this invention is to provide a method and apparatusfor forming longitudinal joints in concrete that is inexpensive, timely,easy, efficient, and which reduces or eliminates maintenance of thejoint.

These and other objects of the invention will be understood by those ofskill in the art upon a review of this specification, the associateddrawings and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a longitudinal joint formed in theconventional manner.

FIG. 2 is a perspective view of a knife member of the present invention,with trowel and vibrator attached.

FIG. 3 is rear perspective of the knife member of FIG. 2 mounted on theunderside of a concrete slip-forming machine.

FIG. 4 is rear perspective of the knife member of FIG. 2 mounted on theunderside of a concrete slip-forming machine and including an optionalrubber bushing mount.

FIG. 5 is a schematic diagram of a possible arrangement of the troweland process of forming a longitudinal joint according to the presentinvention.

FIG. 6 is a plan view showing a hairline crack formed along thelongitudinal joint.

FIG. 7 is a cross-sectional view of a sample of concrete removed fromthe area of a longitudinal joint formed according to the presentinvention showing the aggregate moved to either side of the joint andthe concrete paste strip along which the crack or joint forms.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

In the conventional process for forming a longitudinal joint inconcrete, the concrete is formed and allowed to harden sufficiently thatit will support the weight of a concrete saw, its operator andassociated apparatus. The saw is used to cut a longitudinal groove inthe hardened concrete that is typically between one-eighth andthree-eighths inch wide, as illustrated in FIG. 1. It is important thatthis operation be completed during a window of time in the hardeningprocess of the concrete that may last less than about 12 hours; failureto create the sawn joint within the window is likely to result inuncontrolled cracking of the roadway which requires substantial time andexpense to repair. Cutting the concrete requires the use of an expensiveconcrete saw, frequent replacement of blades, a lubricant, and of coursean operator. Not uncommonly, the operation must be carried out at nightin order to complete it during the optimal window and so lights may alsobe required. A large amount of silicaceous dust is formed (up to 4000lbs. per mile) and the saw is very noisy in operation. The operator mustwear a mask and ear protection.

The sawn groove is cleaned out using a high capacity air compressor andsmall tractor, once again requiring the expense of two additionalmachines, two operators, and resulting in the production of a largeamount of dust. Backer rod is inserted into the cleaned groove about aninch below the surface of the concrete. Hot liquid asphaltic rubber isused to fill the void in the crack above the backer rod. It is common torequire removal and replacement of the sealant at approximatelyseven-year intervals.

There is illustrated in FIG. 2, generally at 10, a knife member of thepresent invention. The knife member 10 includes a vertically disposedknife 12 which extends downwardly from a horizontally disposed float 14.The forward end section 16 of the knife 12 extends forwardly of thefloat 14 and includes a sloped forward edge. The knife 12 and float 14are secured to a mounting frame 18 used to mount the trowel 10 on aconcrete slip-forming machine (not shown). A vibrator 20 is mounted onthe knife member 10 and is used as described below to vibrate the knifemember 10 during operation.

The knife member 10 is illustrated in FIGS. 3 and 4 mounted on theunderside of a concrete slip-forming machine 22. A rearward end portion24 of the knife 12 extends upwardly to form a mounting member for theknife member 10. An optional rubber bushing 26 is attached to the upperend of the rearward end portion 24 and is received about a horizontalmounting pin 28 of the slip-forming machine 22 (FIG. 4). The rubberbushing 26 assists in mechanically isolating vibrations in the knifemember 10 from the slip-forming machine 22 while permitting theslip-forming machine 22 to draw the knife member 10 through the plasticconcrete formed by the machine 22. The knife member 10 is also mountedto the machine 22 at a forward position by the use of bolts or the like(not shown) extending through the openings 28 and 30 in the knife 12(FIG. 2) and through corresponding opening in the slip-forming machine22 (not shown).

It is well known in the art that the aggregate used in the concrete isusually much harder than the Portland cement that fills the spacebetween the aggregate. This is one of the reasons why it is difficult tosaw concrete; the saw must cut through not only the cement, but alsoindividual pieces of aggregate that are present along the path of thesaw. Additionally, when cracks form in concrete, they almost alwaystravel through the cement and around the aggregate. This results in acrack that meanders not only at the surface of the concrete but throughthe depth of the concrete as well. Indeed, one of the reasons that thelongitudinal joints are formed is to define a controlled path for thecrack and avoid cracks in undesired areas of the concrete.

An optional vibrator 20 may be used to vibrate the knife member 10during operation. Vibration assists in reducing the resistance of theknife 12 as it is drawn through the plastic concrete and will assist informing a smoother joint. Moreover, in the process of the presentinvention, the vibration of the knife 12 assists in the importantfunction of pushing aside aggregate that are encountered by the knife 12as it passes through the concrete. If the knife 12 is not vibrated and alarge or irregular aggregate particle is encountered, the knife 12 maypush the aggregate particle ahead of the knife 12, possibly creating avoid in the concrete and abnormally disturbing the surface of theconcrete. Forming a slope in the forward or leading edge of the knife 12and preparing a sharpened or ground leading edge assists in deflectingaggregate downwardly and under the knife 12 and the vibrator 20 bothassists in this process and in deflecting the aggregate to either sideof the knife 12. Another advantage of vibrating the knife member 10 isto assist in consolidating the concrete around the knife 12 and underthe float 16. A principal, unanticipated advantage of the deflection ofthe aggregate below and to either side of the knife 12 is the formationof a vertical strip of concrete paste or cement that is free ofaggregate. This strip is a weakened zone or area of the concrete. Anycrack induced in the area of the strip will follow the weakened stripand be constrained along the strip. Accordingly, the concrete pastestrip performs the function of the sawn groove of the conventionaltechnique in providing a controlled, constrained path for crackformation.

In a preferred embodiment, the vibrator 20 is a hydraulically actuatedvibrator of the type already in common use on concrete slip-formingmachines. Accordingly, operators of concrete slip-forming machines haveexisting inventories of suitable vibrators, including experience withoperating and repairing the vibrators, and the machines themselves havesupplies of pressurized hydraulic fluid, control systems, and supplylines needed to operate the vibrator 20. While too high of a frequencyof the vibrator tends to disrupt the concrete, creating voids, thevibrator 20 may be operated at a wide range of frequencies depending onthe conditions. Most commonly, vibrations in the range of from about 20cycles per second and about 50 cycles per second will producesatisfactory results.

A typical roadway forming operation using the method and apparatus ofthe present invention is illustrated schematically in FIG. 5. A beltplacer 32 places uncured or plastic concrete in the general location ofthe finished roadway. Following the belt placer 32 is the slip-formingmachine or paver 22. The knife member 10 of the present invention is, inthis embodiment, mounted at the rearward or trailing end of the paver22. Behind the paver 22, manually operated bull floats 34 are used tofurther smooth the surface of the formed concrete roadway. Thereafter,tining to roughen the surface of the roadway is done and the roadway isallowed to cure.

In a preferred embodiment, the knife 12 is of a depth to create a stripof concrete paste that is approximately one-fifth to one-half, and morepreferably about one-third, of the thickness of the concrete roadwaybeing formed. The thickness of the knife 12 must not be so large thatthe gap in the concrete does not close with the concrete paste. Theknife preferably is between about one-sixteenth and about one-quarterinch thick and, more preferably, about one-eighth inch thick. The lengthof the knife 12 is believed to have an effect at preventing possible“rebonding” or “aggregate interlock” which may disrupt theaggregate-free strip of cement paste from forming behind the knife 12.That is, if the knife 12 is not sufficiently long, it is possible thatthe aggregate will not have been displaced fully to the side of theknife 12 by the time it has passed and so may drift back in to the areaor zone of the concrete paste strip. In the preferred embodiment, theknife 12 is between about 20 and about 45 inches long and the slope ofthe leading end portion is between about 20 degrees and about 60 degreesrelative to horizontal.

The strip of concrete paste that is formed by the knife 12 isillustrated in FIG. 7 generally at 36, which shows a cross section of asample core taken from a formed roadway. The strip 36 is evident as thesmooth, upper portion of the core sample in the photograph, the smoothsurface indicating that the cross-section was taken through theaggregate-free zone of cement paste created when the aggregate particleswere deflected to the side of the knife. In contrast, the presence ofthe aggregate 38 is clearly seen in the region of the core below thestrip created by the knife.

Within a short amount of time after formation of the roadway and thelongitudinal joint, a hairline crack, shown at 40 in FIG. 6, forms alongthe joint. The hairline crack is usually so tight that no sealing isneeded. Moreover, the hairline crack is in a line very close to parallelto the roadway.

While the foregoing description of a preferred embodiment has focused onconcrete containing aggregate, the method and apparatus will also workin forming a longitudinal joint in fiber-reinforced concrete. In suchconcrete, it may be desired to decrease the angle of attack of theleading edge of the knife 12 to assist in preventing the knife 12 fromcarrying the fibers.

Although the invention has been described with respect to a preferredembodiment thereof, it is to be also understood that it is not to be solimited since changes and modifications can be made therein which arewithin the full intended scope of this invention as defined by theappended claims.

1. A method for forming a longitudinal joint during a concrete formingoperation, comprising the steps of: (a) pulling an upright knife throughplastic concrete that has been at least partially formed into a surface;(b) creating a strip of aggregate-free concrete paste by deflectingaggregate in the concrete to either side of the knife; and (c) allowingthe concrete paste to cure thereby forming a weakened strip in theroadway which constrains crack formation in the surface to the path ofthe knife.
 2. A method as defined in claim 1, further comprising thestep of vibrating the knife to assist in deflection of the aggregate toeither side of the knife.
 3. A method as defined in claim 1, furthercomprising the step of smoothing the concrete surface adjacent eitherside of the joint formed by the knife.
 4. Apparatus for forming alongitudinal joint in a concrete surface being formed by a concreteslip-forming machine, comprising: (a) a knife member mounted on themachine; and (b) a knife extending downwardly from the trowel throughthe surface of the concrete to a predetermined depth; (c) wherein themachine pulls the knife through the concrete to deflect aggregate toeither side of the knife creating a strip of aggregate-free concretepaste which, upon curing of the concrete forms a weakened zone whichconstrains the formation of a crack comprising a longitudinal joint inthe concrete surface.
 5. Apparatus as defined in claim 4, furthercomprising a vibrator mounted on the knife member to assist indeflecting the aggregate to either side of the knife.
 6. Apparatus asdefined in claim 4, further comprising a float for smoothing theconcrete surface to either side of the formed joint.
 7. Apparatus asdefined in claim 4, wherein the knife comprises a leading edge that issloped relative to horizontal between about 20 degrees and about 80degrees.
 8. A method for forming a longitudinal joint during afiber-reinforced concrete forming operation, comprising the steps of:(a) pulling an upright knife through plastic concrete that has been atleast partially formed into a surface; (b) creating a strip offiber-free concrete paste by deflecting fibers in the concrete to eitherside of the knife; and (c) allowing the concrete paste to cure therebyforming a weakened strip in the roadway which constrains crack formationin the surface to the path of the knife.
 9. Apparatus for forming alongitudinal joint in a fiber-reinforced concrete surface being formedby a concrete slip-forming machine, comprising: (a) a trowel mounted onthe machine; and (b) a knife extending downwardly from the trowelthrough the surface of the concrete to a predetermined depth; (c)wherein the machine pulls the knife through the concrete to deflectfibers to either side of the knife creating a strip of fiber-freeconcrete paste which, upon curing of the concrete forms a weakened zonewhich constrains the formation of cracks in the concrete surface to thepath of the knife.